Glass substrate for information recording medium and magnetic disk

ABSTRACT

To provide a glass for an information recording medium substrate, which is excellent in weather resistance. 
     A glass substrate for an information recording medium, which is made of an alkali aluminosilicate glass, wherein the difference obtained by subtracting the glass transition temperature T g  from the fictive temperature T f , i.e. T f −T g , is at most 5° C. The glass substrate for an information recording medium, which comprises, as represented by mol % based on the following oxides, from 64 to 67% of SiO 2 , from 8 to 10% of Al 2 O 3 , from 10 to 13% of Li 2 O, from 9 to 12% of Na 2 O, from 0 to 2% of K 2 O and from 2 to 4% of ZrO 2 , provided that the total content of Li 2 O, Na 2 O and K 2 O, i.e. Li 2 O+Na 2 O+K 2 O, is from 21 to 25%.

TECHNICAL FIELD

The present invention relates to a glass substrate to be used for aninformation recording medium such as a magnetic disk (hard disk), and amagnetic disk.

BACKGROUND ART

As substrates for information recording media, particularly for magneticdisks, glass substrates are widely used, and, for example, a glasscontaining, as represented by mass %, from 47 to 60% of SiO₂, from 8 to20% of Al₂O₃, from 2 to 8% of Na₂O, from 1 to 15% of K₂O, from 1 to 6%of TiO₂ and from 1 to 5% of ZrO₂ has been proposed.

Patent Document 1: WO08/117758

DISCLOSURE OF THE INVENTION Problems to Be Solved by the Invention

A glass substrate for a magnetic disk is required to have an appropriateexpansion coefficient and Young's modulus, and further, it is requiredthat its surface property does not significantly change during storageto prevent films formed on the substrate, such as a base film, amagnetic film and a protective film, from being likely to be peeled,i.e., it is required to have weather resistance.

An object of the present invention is to provide a glass substrate for amagnetic disk having improved weather resistance.

Means to Solve the Problems

The present invention provides a glass substrate for an informationrecording medium, which is made of an alkali aluminosilicate glass,wherein the difference obtained by subtracting the glass transitiontemperature T_(g) from the fictive temperature T_(f), i.e. T_(f)−T_(g),is at most 5° C.

Further, the present invention provides the glass substrate for aninformation recording medium, wherein the alkali aluminosilicate glasshas an alkali metal oxide content of from 15 to 26 mol %.

Further, the present invention provides the glass substrate for aninformation recording medium, which comprises, as represented by mol %based on the following oxides, from 64 to 67% of SiO₂, from 8 to 10% ofAl₂O₃, from 10 to 13% of Li₂O, from 9 to 12% of Na₂O, from 0 to 2% ofK₂O and from 2 to 4% of ZrO₂, provided that the total content of Li₂O,Na₂O and K₂O, i.e. Li₂O+Na₂O+K₂O, is from 21 to 25%. For example,“comprises . . . from 0 to 2% of K₂O” means that K₂O is not essentialbut may be contained in an amount of at most 2%.

Further, the present invention provides a magnetic disk having amagnetic recording layer formed on such a glass substrate for aninformation recording medium.

The weather resistance of a glass substrate for an information recordingmedium depends mainly on its glass composition, but the presentinventors have found that the weather resistance can be improved by adecrease in the fictive temperature even with the same glasscomposition, and they have thus arrived at the present invention.

Effects of the Invention

According to the present invention, a glass for an information recordingmedium substrate, which is excellent in weather resistance can beobtained. And, films formed on the substrate, such as a base film, amagnetic film and a protective film, are thereby made hardly likely tobe peeled.

BEST MODE FOR CARRYING OUT THE INVENTION

The density d of the glass (hereinafter referred to as the glass of thepresent invention) of the glass substrate for an information recordingmedium of the present invention (hereinafter referred to as the glasssubstrate of the present invention) is preferably at most 2.60 g/cm³. Ifit exceeds 2.60 g/cm³, a load is applied to a motor during rotation of adrive, and thus it is possible that the power consumption is increasedor rotation of the drive becomes unstable. It is preferably at most 2.54g/cm³.

The glass of the present invention preferably has a Young's modulus E ofat least 76 GPa. If it is less than 76 GPa, warpage or deflection islikely to occur during rotation of the drive, and it may possibly becomedifficult to obtain an information recording medium having a highrecording density. E is more preferably at least 77 GPa.

The glass of the present invention preferably has a specific modulus E/dof at least 28 MNm/kg. If E/d is less than 28 MNm/kg, warpage ordeflection is likely to occur, and it may possibly be difficult toobtain an information recording medium having a high recording density.E/d is more preferably at least 30 MNm/kg.

The glass of the present invention preferably has a glass transitionpoint T_(g) of at least 450° C. If it is less than 450° C., the magneticlayer forming-heat processing temperature cannot be made sufficientlyhigh, and it may possibly be difficult to increase the coercive force ofthe magnetic layer. It is more preferably at least 460° C.

The glass of the present invention preferably has an average linearexpansion coefficient a of at least 56×10⁻⁷/° C. within a range of from−50 to 70° C. If it is less than 56×10⁻⁷/° C., the difference from thethermal expansion coefficient of another component such as a drive madefrom a metal becomes large, and thus a substrate may possibly be easilybroken due to generation of a stress at the time of a temperaturechange. It is more preferably at least 58×10⁻⁷/° C. a is typically atmost 100×10⁻⁷/° C.

Next, the glass of the present invention will be described using acontent represented by mole percentage.

The glass of the present invention is an alkali aluminosilicate glass,and typically, it has a SiO₂ content of from 61 to 71%, an Al₂O₃ contentof from 7 to 17% and an alkali metal oxide content of from 15 to 26%.

If SiO₂ is less than 61%, the acid resistance will be decreased, d willbecome large, or the liquid phase temperature will be raised, and theglass will become unstable. If it exceeds 71%, temperature T₂ where theviscosity becomes 10² dPa·s and temperature T₄ where the viscositybecomes 10⁴ dPa·s will be raised, and melting and forming of the glasswill be difficult, E or E/d will be decreased, or a will become small.

If Al₂O₃ is less than 7%, the weather resistance will be decreased, E orE/d will be decreased, or T_(g) will be lowered. If it exceeds 17%, theacid resistance will be decreased, T₂ and T₄ will be raised, and thusmelting and forming of the glass will be difficult, a will become small,or the liquid phase temperature will be too high.

As the alkali metal oxide, Li₂O, Na₂O or K₂O is common, but if the totalcontent of alkali metal oxides is less than 15%, a will become small, ormelting performance of the glass will be declined. If it exceeds 26%,the weather resistance will be decreased.

It is preferred that Li₂O is from 6 to 16%, Na₂O is from 2 to 13%, andK₂O is from 0 to 8%.

If Li₂O is less than 6%, it may be possible that a becomes small, ormelting performance of the glass will be declined. If it exceeds 16%,the weather resistance or T_(g) may possibly be decreased.

If Na₂O is less than 2%, it may be possible that a becomes small, ormelting performance of the glass will be declined. If it exceeds 13%,the weather resistance or T_(g) may possibly be decreased.

K₂O is not essential, but may be contained in an amount of at most 8% inorder to increase a or improve the melting performance of the glass. Ifit exceeds 8%, it may be possible that the weather resistance isdecreased, or E or E/d will be decreased.

This alkali aluminosilicate glass may contain components other SiO₂,Al₂O₃ and alkali metal oxides in an amount within a range not to impairthe properties as a substrate for an information recording medium, butthe content of such components is typically at most 8% in total.

As one of preferred embodiments of the glass of the present invention, aglass comprising from 64 to 67% of SiO₂, from 8 to 10% of Al₂O₃, from 10to 13% of Li₂O, from 9 to 12% of Na₂O, from 0 to 2% of K₂O and from 2 to4% of ZrO₂, provided that the total content of Li₂O, Na₂O and K₂O, i.e.Li₂O+Na₂O+K₂O, is from 21 to 25%, may be mentioned (this glass willhereinafter be referred to as glass A of the present invention).

Next, the composition of glass A of the present invention will bedescribed

SiO₂ is a component to form the skeleton of the glass and is essential.If it is less than 64%, the acid resistance will be decreased, d willbecome large, or the liquid phase temperature will be raised and theglass will become unstable. If it exceeds 67%, T₂ and T₄ will be raisedand melting and forming of the glass will become difficult, E or E/dwill be decreased, or a will become small.

Al₂O₃ has an effect to increase weather resistance, and is essential. Ifit is less than 8%, such an effect will be small, E or E/d will bedecreased, or T_(g) will be lowered. If it exceeds 10%, the acidresistance will be decreased, T₂ and T₄ will be raised and melting andforming of the glass will become difficult, a will become small or theliquid phase temperature will be too high.

Li₂O has an effect to increase E, E/d or a, or to improve the meltingperformance of the glass, and is essential. If it is less than 10%, suchan effect will be small. If it exceeds 13%, the weather resistance willbe decreased, or T_(g) will be lowered.

Na₂O has an effect to increase a or to improve melting performance ofthe glass, and is essential. If it is less than 9%, such an effect willbe small. If it exceeds 12%, the weather resistance will be decreased,or T_(g) will be lowered.

K₂O is not essential, but it has an effect to increase a, or to improvemelting performance of the glass, and it may be contained in an amountof at most 2%. If it exceeds 2%, the weather resistance will bedecreased, or E or E/d will be decreased. In the case where K₂O iscontained, its content is preferably at least 0.1%.

If the total content of Li₂O, Na₂O and K₂O, i.e. Li₂O+Na₂O+K₂O(hereinafter referred to as R₂O), is less than 21%, a will become small,or the melting performance of the glass will be declined. If R₂O exceeds25%, the weather resistance will be decreased.

ZrO₂ has an effect to increase E, E/d or T_(g), to increase the weatherresistance or to improve melting performance of the glass, and thus itis essential. If it is less than 2%, such an effect will be small. If itexceeds 4%, it may be possible that d becomes large or the liquid phasetemperature becomes too high.

The glass of the present invention consists essentially of the abovecomponents, but it may contain other components within a range not toimpair the object of the present invention. In such a case, the contentof the other components is, in total, preferably at most 2%, morepreferably at most 1%, particularly preferably at most 0.5%.

Next, examples of components other than the above components will bedescribed.

MgO is not essential, but it has an effect to increase E, E/d or a, tomake the glass being hardly scratched, or to improve melting performanceof the glass, while maintaining the weather resistance, and it may becontained in an amount of at most 2%. If it exceeds 2%, the liquid phasetemperature will be too high. It is more preferably at most 1%,particularly preferably at most 0.5%. No MgO is typically contained.

CaO is not essential, but it has an effect to increase a or to improvemelting performance of the glass, while maintaining the weatherresistance, and it may be contained in an amount of at most 2%. If itexceeds 2%, it may be possible that d becomes large, E is decreased, orthe liquid phase temperature becomes too high. It is more preferably atmost 1%, particularly preferably at most 0.5%. No CaO is typicallycontained.

SrO may be contained in an amount of at most 2% in order to increase aor to improve melting performance of the glass. If it exceeds 2%, it maybe possible that d becomes large or the glass becomes likely to bescratched. It is more preferably at most 1%, particularly preferably atmost 0.5%. No SrO is typically contained.

BaO may be contained in an amount of at most 2% in order to increase aor to improve melting performance of the glass. If it exceeds 2%, it maybe possible that d becomes large or the glass becomes likely to bescratched. It is more preferably at most 1%, particularly preferably atmost 0.5%. No BaO is typically contained.

TiO₂ may be contained in an amount of less than 2% in order to increaseE, E/d or T_(g), to increase weather resistance, etc. If it is 2% ormore, T_(L) may possibly be too high, or phase separation may possiblybe likely to occur. It is more preferably at most 1%, particularlypreferably at most 0.5%. No TiO₂ is typically contained.

B₂O₃ may be contained in an amount of at most 2% in order to increase Eor E/d, to increase weather resistance, to improve melting performanceof the glass, etc. If it exceeds 2%, phase separation may possibly belikely to occur. It is more preferably at most 1%, particularlypreferably at most 0.5%. No B₂O₃ is typically contained.

La₂O₃ may be contained in order to e.g. improve E while maintaining theweather resistance, but in such a case, it is preferably at most 2%. Ifit exceeds 2%, it may be possible that d becomes large or the liquidphase temperature becomes too high. It is more preferably at most 1%,particularly preferably at most 0.5%. No La₂O₃ is typically contained.

Nb₂O₅ may be contained in order to e.g. improve E while maintaining theweather resistance, but in such a case, it is preferably at most 2%. Ifit exceeds 2%, it may be possible that d becomes large or the liquidphase temperature becomes too high. It is more preferably at most 1%,particularly preferably at most 0.5%. No Nb₂O₅ is typically contained.

RE₂O₃ may be contained in an amount of less than 1% in total. The RE₂O₃represents an oxide of a rare earth selected from the group consistingof Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, anda mixture thereof.

A clarifier such as SO₃, Cl, As₂O₃, Sb₂O₃ or SnO₂ may be contained in anamount of up to 2% in total.

A colorant such as Fe₂O₃, Co₃O₄ or NiO may be contained in an amount ofup to 2% in total.

Another preferred embodiment of the glass of the present invention maybe, for example, one which contains from 64 to 69% of SiO₂, from 9 to11% of Al₂O₃, from 6 to 9% of Li₂O, from 9 to 13% of Na₂O, from 0 to 2%of K₂O, from 0 to 4% of MgO, from 1 to 5% of CaO and from 0 to 2% ofZrO₂, provided that Li₂O+Na₂O+K₂O is from 16 to 20%; one which containsfrom 66 to 71% of SiO₂, from 7 to 9% of Al₂O₃, from 0 to 3% of B₂O₃,from 12 to 16% of Li₂O, from 2 to 5% of Na₂O, from 0 to 3% of K₂O, from0 to 5% of MgO, from 0 to 3% of TiO₂, from 0 to 2% of ZrO₂, from 0 to 2%of La₂O₃ and from 0 to 2% of Nb₂O₅, provided that Li₂O+Na₂O+FK₂O is from16 to 21%; and one which contains from 61 to 66% of SiO₂, from 11.5 to17% of Al₂O₃, from 8 to 16% of Li₂O, from 2 to 8% of Na₂O, from 2.5 to8% of K₂O, from 0 to 6% of MgO, from 0 to 4% of TiO₂ and from 0 to 3% ofZrO₂, provided that Al₂O₃+MgO+TiO₂ is at least 12% and Li₂O+Na₂O+K₂O isfrom 16 to 23%, and that the content of B₂O₃ is less than 1% if it iscontained.

The glass plate of the present invention is typically a circular glassplate.

The weather resistance of the glass substrate of the present inventionis evaluated by C_(R) defined by C_(R)═C_(Li)+C_(Na)+C_(K), whereinC_(Li), C_(Na) and C_(K) are amounts of Li, Na and K, respectively,which are precipitated on the surface of the glass after it is holdunder a water-vapor atmosphere at 120° C. at 0.2 MPa for 20 hours.

The difference obtained by subtracting the glass transition temperatureT_(g) from the fictive temperature T_(f), i.e. T_(f)−T_(g), of the glasssubstrate of the present invention is at most 5° C. in order to improvethe weather resistance. If it exceeds 5° C., the effect to improveweather resistance will be hardly obtained. T_(f)−T_(g) is preferably atmost −10° C., but if it is at most −25° C., this effect will be moredistinct. T_(f)−T_(g) is typically at least −35° C.

In a case where the glass of the present invention is the above glass A,C_(R) of the glass substrate of the present invention is preferably atmost 7 nmol/cm². If C_(R) exceeds 7 nmol/cm², films formed on thesubstrate, such as a base film, a magnetic film and a protective film,will be likely to be peeled.

The glass substrate of the present invention is typically used as aglass substrate for a magnetic disk.

Glass substrates for magnetic disks are widely used for 2.5-inchsubstrates (outside diameter of a glass substrate: 65 mm) used for e.g.notebook PCs, 1.8-inch substrates (outside diameter of a glasssubstrate: 48 mm) used for e.g. portable MP3 players and so on. Theirmarket is growing every year, and on the other hand, they are desired tobe supplied at a low price. Glass used for such glass substrates ispreferably suitable for mass production.

A plate glass is widely mass-produced by continuous molding methods suchas a float process, a fusion method and a down-draw method, and theglass of the present invention includes a glass which is able to beformed by e.g. a float process and thus it is suitable for massproduction.

The melting and forming method for the glass substrate of the presentinvention is not particularly limited, and various methods can beapplied. For example, materials for the respective components commonlyused are blended to have a desired composition, and such a mixture isheated and melted in a glass melting furnace. Then, the glass ishomogenized by e.g. bubbling, stirring or addition of a clarifier,followed by forming into a plate glass having a predetermined thicknessby a known method such as a down-draw method including a fusion method,a float process or a press method, and then, after annealing, processingsuch as grinding or polishing is carried out as necessary, to obtain aglass substrate having prescribed size and shape. As the forming method,particularly, a float process suitable for mass production is preferred.Further, a continuous forming method other than a float process, i.e. afusion method or a down-draw method, is also suitable.

A method for producing the glass substrate of the present invention ispreferably such that, in cooling of the glass in the last step where theglass temperature becomes at least its strain point, the time t duringwhich the glass temperature is at least its strain point and at most atemperature where the glass viscosity becomes 10¹⁰ dPa·s is at least 13minutes. If the time t is less than 13 minutes, it may be possible thatit becomes difficult to bring the above T_(f)−T_(g) to at most 5° C. Itis more preferably at least 18 minutes.

EXAMPLES

Materials were prepared so that a glass having a composition comprising,as represented by mol %, 65.7% of SiO₂, 8.5% of Al₂O₃, 12.4% of Li₂O,10.9% of Na₂O and 2.5% of ZrO₂ would be obtained, and they were meltedat a temperature of from 1,550 to 1,600° C. for 3 to 5 hours using aplatinum crucible. In the melting, a platinum stirrer was put into themolten glass, and the glass was stirred for 2 hours to be homogenized.Then, the molten glass was cast to form a plate and annealed to roomtemperature at a cooling rate of 1° C./min.

Thus obtained plate glass had a density d of 2.51 g/cm³, an averagelinear expansion coefficient a of 77×10⁻⁷/° C., a Young's modulus E of84 GPa, a specific modulus E/d of 33.7 MNm/kg, a glass transition pointT_(g) of 494° C., a fictive temperature T_(f) of 478° C., a strain pointT_(Str) of 457° C. at which the glass viscosity becomes 10^(14.5) dPa·sand a temperature T₁₀ of 580° C. at which the glass viscosity becomes10¹⁰ dPa·s. These measurements were carried out by the methods describedbelow.

d: d was measured by an Archimedes method using from 20 to 50 g of aglass having no bubble in it.

a: By means of a differential thermal dilatometer and using quartz glassas a reference sample, the rate of elongation of a glass when it washeated at a rate of 5° C./min from room temperature to a temperature atwhich the glass is softened and elongation is no longer observed, i.e.to an yield point, was measured, and from the obtained thermal expansioncurve, an average linear expansion coefficient within a range of −50 to70° C. was calculated.

E: With respect to a glass plate having a thickness of from 5 to 10 mmand a size of 3 cm square, the measurement was carried out by anultrasonic pulse method.

T_(g): By means of a differential thermal dilatometer and using quartzglass as a reference sample, the rate of elongation of a glass when itwas heated at a rate of 5° C./min from room temperature to its yieldpoint, and the temperature corresponding to the critical point in theobtained thermal expansion curve was taken as the glass transitionpoint.

T_(Str): The strain point was measured in accordance with “Determinationof annealing point and strain point by fiber elongation” (JIS R3103-2:2001).

T₁₀: A temperature-viscosity curve within a viscosity range of from10^(1.5) dPa·s to 10^(4.5) dPa·s was measured with a rotationviscometer. Further, a softening point at which the viscosity becomes10^(7.6) dPa·s was measured in accordance with “Determination ofsoftening point” (JIS R 3103-1:2001), and an annealing point at whichthe viscosity becomes 10¹³ dPa·s was measured in accordance with“Determination of annealing point and strain point” (JIS R 3103-2:2001).A temperature-viscosity curve including these viscosity ranges wasobtained by fitting using Fulcher's equation based on the abovetemperature-viscosity curve, the softening point, the annealing pointand the strain point, and T₁₀ was obtained from thistemperature-viscosity curve.

The fictive temperature T_(f) was measured by the following method.

First, the above plate glass was processed into a glass plate having athickness of 0.4 mm and a size of 1 cm square. This glass plate wasplaced in a box-shaped electric furnace, and the temperature was raisedto 630° C. It was maintained at 630° C. for 10 minutes, and then cooledto a holding temperature T_(k) at a cooling rate of 1° C./min by aprogram control. After it was maintained at T_(k) for 140 hours, thesample was retrieved from the electric furnace and quenched to roomtemperature under the air atmosphere. T_(k) was set to be 510° C., 500°C., 490° C., 480° C. and 450° C. The thickness of the glass issufficiently thin, and thus each T_(f) of the glass becomes T_(k). Therefractive index of each sample was measured, and a calibration curve ofT_(f) and the refractive index was prepared.

Next, the above plate glass was processed into a glass plate having athickness of 1.2 mm and a size of 4 cm square. The glass plate wasplaced into a box-shaped electric furnace, and the temperature wasraised to 630° C. It was maintained at 630° C. for 10 minutes, and thentwo types of samples were prepared, for which the cooling rates were setto be 0.1° C./min and 1° C./min, respectively, by a program control (theformer is identified as Example 1, the latter as Example 2). Therefractive index of each sample was measured, and T_(f) was obtainedusing the above calibration curve. The results were 463° C. for Example1 and 478° C. for Example 2.

Further, the above plate glass was processed into a glass plate having athickness of 1.2 mm and a size of 4 cm square. This glass plate wasflowed through a belt conveyer type electric furnace, and two types ofsamples i.e. Example 3 and Example 4 were prepared, which have differentcooling histories by controlling of the belt velocity. That is, theelectric furnace has a length of 3.4 m and has five heaters installed at0.3 m, 1 m, 1.7 m, 2.4 m and 3 m away from the inlet, and the presettemperatures of the five heaters were set to be, in ascending order ofdistance from the inlet, 350° C., 450° C., 520° C., 610° C. and 630° C.;and a sample for which the belt velocity was 11 mm/min was identified asExample 3, and a sample for which the belt velocity was 94 mm/min wasidentified as Example 4. The refractive index of each sample wasmeasured and T_(f) was obtained by using the above calibration curve.The results were 497° C. for Example 3 and 513° C. for Example 4.

In the preparation of a sample using such a belt conveyer type electricfurnace, there was concern that T_(f) of a sample varies depending onthe position, and thus the glass plate was divided into nine equal partshaving a thickness of 1.2 mm and a size of 1.2 cm square, and therefractive index of each part was measured. As a result, it was foundthat the refractive index of all parts were equal and T_(f) of all partshave no difference. Further, the center part among the nine parts wasdivided in the thickness direction into three parts and the refractiveindex of each part was measured, and it was found that the refractiveindex of all parts were equal and T_(f) of all parts have no difference.Thus, it was confirmed that T_(f) of a glass plate having a thickness of1.2 mm and a size of 4 cm square is uniform in the whole plate.

T_(f)−T_(g) of the Examples were −32° C. for Example 1, −17° C. forExample 2, 2° C. for Example 3 and 19° C. for Example 4, whereinExamples 1 to 3 are Working Examples of the present invention andExample 4 is a Comparative Example. T_(f)−T_(g) of a glass plate whichis continuously industrially produced by forming a glass into a plate bya float process, a down-draw method or the like and then an annealingstep is at least 15° C., and Example 4 is a model of such an industrialproduct.

Weather resistance index C_(R) of the glass plates of Examples 1 to 4were measured as described below, and the results were 6.2 nmol/cm² forExample 1, 6.9 nmol/cm² for Example 2, 6.9 nmol/cm² for Example 3 and7.3 nmol/cm² for Example 4. Thus, it is found that by permittingT_(f)−T_(g) to be at most 5° C., the weather resistance of a glass isimproved even if the composition is the same.

C_(R): Both surfaces of a glass plate having a thickness of from 1 to 2mm and a size of 4 cm×4 cm was subjected to mirror polishing usingcerium oxide and washed using calcium carbonate and a neutral detergent,and then the glass plate was placed in a highly accelerated stress testchamber (unsaturated-type pressure cooker EHS-411M, manufactured byESPEC Corp.) and was statically left under a water-vapor atmosphere at120° C. at 0.2 MPa for 20 hours. The tested sample and 20 ml ofultrapure water were put into a washed reclosable poly bag and a surfaceprecipitate was dissolved in a ultrasonic bath for 10 minutes, and thequantity of eluted substances of respective alkaline components (Li andNa) was determined using ICP-MS. The eluted amounts of respectivealkaline components were converted to moles and were normalized by thesurface area of the sample, and their sum was regarded as C_(R).

INDUSTRIAL APPLICABILITY

The present invention is useful for production of an informationrecording medium such as a magnetic disk and for a glass substrate foran information recording medium.

The entire disclosure of Japanese Patent Application No. 2009-265079filed on Nov. 20, 2009 including specification, claims and summary isincorporated herein by reference in its entirety.

1. A glass substrate for an information recording medium, which is madeof an alkali aluminosilicate glass, wherein the difference obtained bysubtracting the glass transition temperature T_(g) from the fictivetemperature T_(f), i.e. T_(f)−T_(g), is at most 5° C.
 2. The glasssubstrate for an information recording medium according to claim 1,wherein the alkali aluminosilicate glass has an alkali metal oxidecontent of from 15 to 26 mol %.
 3. The glass substrate for aninformation recording medium according to claim 1, which comprises, asrepresented by mol % based on the following oxides, from 64 to 67% ofSiO₂, from 8 to 10% of Al₂O₃, from 10 to 13% of Li₂O, from 9 to 12% ofNa₂O, from 0 to 2% of K₂O and from 2 to 4% of ZrO₂, provided that thetotal content of Li₂O, Na₂O and K₂O, i.e. Li₂O+Na₂O+K₂O, is from 21 to25%.
 4. The glass substrate for an information recording mediumaccording to claim 1, wherein T_(f)−T_(g) is at most −25° C.
 5. Amagnetic disk having a magnetic recording layer formed on the glasssubstrate for an information recording medium as defined in claim 1.